The invention relates to a method and an apparatus for treating wastewater from a chemical-mechanical polishing process used in chip fabrication.
Chemical-mechanical polishing processes (CMP) are used in semiconductor chip fabrication to planarize the semiconductor wafer or to keep it planar. This involves treating the wafer with a polishing tool with the addition of a polishing fluid, the so-called slurry. Typically, deionized water with a very high purity serves as a basis for the polishing fluid, which is added to chemical additives and/or particles having an abrasive effect.
After the chemical-mechanical polishing process, the polishing fluid running off will, as well as its constituents, additionally contain abraded material from the polishing process and further contaminants. As a result, the polishing fluid is laden with a series of particles having an abrasive effect and a series of chemically active substances. In the case of a wastewater load of about 30 m3/h, the following constituents may be present, for example, in the polishing fluid.
Up till now, the wastewater from a chemical-mechanical polishing process used in chip fabrication has been neutralized chemically, coarse particle contamination is removed by sedimentation, and the wastewater thus treated is passed to the public sewage system and is thus lost for operational purposes.
The known procedure has the drawback that water consumption is considerable; in relatively large chip fabrication plants it is a few m3/h. This amount of water discharged into the public sewage system must be replaced by fresh deionized water of a very high purity, thus entailing corresponding costs of providing the deionized water. Moreover, the wastewater coming from a chemical-mechanical polishing process in chip fabrication causes an additional load to the water treatment plants of the public sewage system, which is environmentally undesirable.
It is accordingly an object of the invention to provide a method and an apparatus for treating wastewater from a chemical-mechanical polishing process used in chip fabrication that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which permit a reduction in the total amount of wastewater produced and to be discharged. In particular, it is desirable, as a development of the invention, for the treated wastewater to be capable of being recycled subsequently to produce deionized water.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for treating wastewater from a chemical-mechanical polishing process used in chip fabrication, the wastewater containing particles having an abrasive effect and chemically active substances attacking the particles having the abrasive effect, which includes removing rapidly the particles having the abrasive effect from the wastewater via ultra-filtration in an ultra-filtration facility such the particles are not substantially dissolved by the chemically active substances present in the wastewater and resulting in ultrafiltrated wastewater.
The use of the ultra-filtration facility allows the wastewater from a chemical-mechanical polishing process (hereinafter simply referred to as CMP wastewater) to be regenerated in such a way as to allow the treated wastewater to be reused within the plant or to be passed to the public sewage system without significant pollution. Ultra-filtration for the purpose of the patent application results in that particles having a diameter of more than 0.4 xcexcm are essentially filtered out. Moreover, it is preferred for particles having a diameter of more than 0.1 xcexcm to be essentially filtered out by the ultra-filtration. In the case of relatively low to medium particle contamination, ultra-filtration is adequate to achieve such a degree of purity of the treated wastewater as to allow the treated wastewater to be subsequently recycled to produce deionized water.
In the process, the CMP wastewater is preferably subjected so rapidly to ultra-filtration, that the particles having an abrasive effect, for example the SiO2 particles, which are present in the CMP wastewater are essentially not dissolved by the chemically active substances, for example KOH, present in the CMP wastewater. If this is ensured, the substances present in the particles can be almost completely removed from the CMP wastewater by the ultra-filtration.
It is further preferred for the particles filtered out in the ultra-filtration facility to be removed from the ultra-filtration facility by back flushing. In the process, it is moreover preferred for the back flushing to occur at sufficiently short intervals for essentially no reaction in the ultra-filtration facility to take place between the particles retained in the ultra-filtration facility and the chemically active substances present in the CMP wastewater.
In the case of organic or inorganic contaminants it may be necessary also to provide a reverse-osmosis stage and/or a nano-filtration stage in addition to the ultra-filtration stage. Reverse osmosis here essentially serves to remove organic-chemistry carbon compounds from the CMP wastewater to be treated. Nano-filtration for the purpose of this patent application results in that particles having a diameter of more than 0.05 xcexcm are essentially filtered out. Moreover, it is preferred for particles having a diameter of more than 0.01 xcexcm to be essentially filtered out by the nano-filtration.
It is particularly economical for the ultra-filtration of the wastewater to be treated to be followed by the measurement of one or more parameter values of the ultrafiltrated wastewater. The ultrafiltrated wastewater, depending on the parameter values measured, to be either passed on directly or passed to a reverse-osmosis facility or a nano-filtration facility or both. Thus, the further, more expensive cleaning stages of reverse osmosis and nano-filtration are used only if they are actually required according to the parameter values measured. Relevant parameter values include, in particular, the conductivity of the treated wastewater, its total content of organic carbon, its particle content or its level of silicon oxides or ammonia NH3.
Preferably, the purified wastewater is subsequently again fed to a regeneration facility for producing deionized water, thus resulting in a closed water cycle.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for treating wastewater from a chemical-mechanical polishing process used in chip fabrication, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.